Japanese Patent Application No. H11-280826 discloses a hydraulic shock absorber in which a vehicle body side sleeve and a wheel side tube are slidably and engagably fitted to each other, a damper cylinder is positioned to stand upright in the wheel side tube, a piston rod, which stands upright in the wheel side tube, is inserted in the damper cylinder, a rod side oil chamber and a piston side oil chamber are formed in the damper cylinder by a piston provided at the piston rod, a damping force generator is provided at the piston, and a pressure side damping force generator is provided in the piston side oil chamber of the damper cylinder. In such a hydraulic shock absorber, oil, used for cushioning, is pressurized by a coil spring, to restrain foaming of the oil and to stabilize a damping effect.
However, by using the coil spring to pressurize oil, a certain degree of pressure is applied to from an initial stroke, such that an increase in the rate of the coil spring is required and a relatively large preset is also required. This allows a pressurization amount at the time of cushion stroke to be increased and, consequently, a repelling force might be changed thereby impairing ride comfort. Moreover, due to an increase in weight and complication of the structure by the coil spring, cost is increased.
The present invention has been made to overcome the drawbacks of existing hydraulic shock absorbers. Accordingly, it is an object of the present invention to provide a hydraulic shock absorber which can stably pressurize cushion oil, is light-weight, has a simplified structure and facilitates improvement in the ride comfort to a driver.